1. Field of the Invention
The present invention relates to an electrochemical apparatus, such as a solid oxide fuel cell apparatus.
2. Description of the Related Art
Since a single fuel cell generates a voltage of about 1 V, in order to generate large output, a plurality of fuel cells must be stacked. In this connection, difficulty is encountered in ensuring a stable stack structure while increasing the number of stacked cells for generating large output.
According to WO2007/029860 A1, particularly FIG. 14 therein, an electrochemical cell made of ceramic is configured such that a fuel flow channel is formed within a fuel electrode, and a solid electrolyte membrane and an air electrode membrane are formed on the fuel electrode. Gas supply holes and gas discharge holes are formed in the cell, and a plurality of the cells are stacked directly on one another, thereby forming a stack. In formation of the stack, the gas supply holes of the adjacent cells are connected to each another to form gas supply channels, whereas the gas discharge holes of the adjacent cells are connected to one another to form gas discharge channels.
According to the stack (a set of stacked cells) described in WO2007/029860 A1, the cells each having a gas flow channel are attached to respective fixing members, and the resultant assemblies are stacked. Since the cells of the stack structure also serve as structural members, stress is apt to be imposed thereon. Particularly, the cell having the gas flow channel therein is weaker in structural strength than a cell having no gas flow channel therein; thus, a stack structure is desirably such that stress is not imposed on the cells.
The applicant of the present invention has disclosed, in PCT JP2008/056636 and “Electricity-generating characteristics of flow-channel-incorporated cell stack,” Proceedings of The 15th SOFC Research Meetings, The SOFC Research Institute, Dec. 5, 2006, pp. 212-215, a stack structure in which an electrochemical cell has a fuel gas flow channel formed therein, and a plurality of the electrochemical cells are supported by gas supply members and gas discharge members while being spaced apart from each other. The flat cells are stacked in such a manner that each of ring-shaped connection members intervenes between the adjacent cells, so that the connection members and the cells are alternatingly arranged in layers, thereby forming a fuel gas or oxidizing gas flow channel.
Also, the applicant of the present invention has disclosed, in Japanese Patent Application No. 2007-324508, a stack in which a plurality of interconnectors are stacked such that the interconnectors accommodate respective flow-channel-incorporated cells. An electrically conductive portion is formed on the surface of each of the cells, and the electrical conductive portions of the cells are electrically connected in series to the respectively adjacent interconnectors. The flat cells are stacked in such a manner that each of ring-shaped connection members intervenes between the adjacent cells, so that the connection members and the cells are alternatingly arranged in layers, thereby forming a fuel gas or oxidizing gas flow channel.
However, when a stack composed of a large number of stacked cells is operated at a high temperature of, for example, 800° C., in some cases, the cracking of some cells has been experienced, with a resultant drop in output per unit volume. Conceivably, this is for the following reason: when a cell is warped or deformed, excessive stress is imposed on some cells, particularly those located in a lower region of the stack; as a result, fine cracking or separation of the cells arises, thereby causing leakage of gas, which leads to a drop in output of generated electricity. Additionally, since the warpage of a cell causes a failure to hold the horizontality of the upper and lower surfaces of the associated connection member, there has been involved a problem of failure to hold a constant cell-to-cell distance.